1. Field of the Invention
The present invention relates to a crushable body strength adjusting system in a vehicle, comprising a device which is disposed between a bumper beam and a vehicle body frame, can be crushed upon reception of a collision load in the longitudinal direction of the vehicle body, and has a variable crush-strength.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2006-8106 discloses a vehicle body strength adjusting system comprising: a bumper beam mounted at front ends of front side frames of an automobile, the bumper beam including a pair of front and rear beam members disposed parallel to each other; and a plurality of variable crush-strength devices disposed at equal distances between the beam members.
Each of the variable crush-strength devices includes: a plurality of buckling plates made of shape memory alloy and disposed adjacent to one another such that they are buckled by a collision shock; and an actuator which switches over the buckling plates between a state in which they are integrally connected to each other and a state in which they are separated from each other. In the state in which the buckling plates are integrally connected to each other, the buckling strength is increased in order to increase the absorbable shock energy. In the state in which the buckling plates are separated from each other, the buckling strength is decreased in order to decrease the absorbable shock energy.
In general, the bumper beam has a shape such that its left and right opposite ends are inclined to extend around along sides of a vehicle body. Therefore, in a system in which inclined left and right ends of a bumper beam are supported at front ends of left and right front side frames by variable crush-strength devices interposed therebetween, when a frontal collision load is input to the bumper beam, the inclined left and right ends are deformed so as to extend straight, whereby the lateral length of the bumper beam is increased. Thus, the variable crush-strength devices fall down and are inclined outward of the vehicle body, leading to a possibility that the collision load being input from the front cannot be effectively absorbed.
In the case where a load acting in a longitudinal direction of the vehicle body is input to the bumper beam upon frontal collision of a vehicle, the buckling plates of the variable crush-strength devices are buckled without laterally falling down, thereby exhibiting a shock-absorbing effect. However, in the case where a load acting in a direction inclined with respect to the longitudinal direction of the vehicle body is input to the bumper beam upon diagonal collision of a vehicle, the buckling plates of the variable crush-strength devices fall down in right and left directions without being buckled, leading to a possibility that a sufficient shock-absorbing effect cannot be exhibited.
In order to effectively absorb a load collision on the vehicle, it is desirable to maintain a high load level for a period from the beginning to the end of the crushing of the buckling plates. However, if a buckling plate is formed of an iron material or an aluminum material, the load steeply increases with an increase in the amount of deformation of the buckling plate to reach a peak and then gradually decreases, leading to a difficulty in maintaining a constantly high load level. On the other hand, if a buckling plate is formed of shape memory alloy, the load steeply increases with an increase in the amount of deformation of the buckling plate to reach a peak and then steeply decreases to reach a bottom, and further gradually increases from the bottom, also leading to a difficulty in maintaining a constantly high load level.